1. Technical Field
The present invention relates generally to pellicles used to protect a mask and, more particularly, to pellicle distortion reduction.
2. Related Art
Pellicles are thin, optically transparent membranes that are used in photolithography to protect patterned photomask surfaces from contamination by airborne particles. The pellicles are conventionally attached to the photomask by a metal frame. Conventional pellicles are made of organic or fluorocarbon polymers. Conventional pellicles are so thin (approximately 0.5 to 2.0 microns) that they do not introduce any appreciable optical distortion to the light passing through them, even if the pellicle is physically distorted from an ideal flat shape.
An emerging photolithography technique is the use of a 157 nanometer (nm) exposure wavelength. Current pellicle technology used at 365 nm, 248 nm, and 193 nm exposure wavelengths are either not transparent enough or not durable enough for the radiation exposure used at the 157 nm exposure wavelength. Unfortunately, no polymers have been found with sufficient radiation durability to be used as pellicles at the 157 nm exposure wavelength. For this reason, thick or hard plate pellicles have been proposed for use at this exposure wavelength. A thick plate pellicle may include a flat, polished piece of fused silica, typically 100 to 1000 microns thick. A drawback of these thicker pellicles is that optical distortions can be induced in the light passing through them more readily than with thinner pellicles. In particular, any bending of a thick plate pellicle will induce severe displacements of the images being transmitted.
Distortion of a pellicle can be provoked by three general causes: gravity, aerodynamic turbulence caused by movement, and forces exerted during acceleration. FIG. 1 illustrates a pellicle 10 coupled to a mask 12 by a pellicle mounting frame 14. As shown, gravity causes the pellicle to sag, which introduces optical distortion to the mask pattern. This sag occurs regardless of whether the pellicle is below the mask, as shown, or above the mask. Sag also occurs when the pellicle and mask are supported substantially vertically, as shown in FIG. 2.
With regard to aerodynamic turbulence, almost all modern lithographic exposure tools use the “step-and-scan” exposure method in which the mask and wafer are scanned at high speed during the exposure. The direction of movement is shown in FIGS. 1 and 2 by the arrows. Mask speeds can reach, for example, over 2 meters per second in a direction parallel to the mask surface. At these speeds, turbulent air flow is likely to be induced by the edges of the mask and the pellicle mounting frame. This turbulent flow can make the pellicle flutter sufficiently to induce image distortions during the exposure. In addition, forces exerted during acceleration during the “step-and-scan” exposure method can make the pellicle flutter and induce image distortion.
A number of remedies to the above-identified problems have been offered. One suggestion is to increase the pellicle stiffness by thickening to reduce the sag and movement in response to turbulence. Thick plate pellicles were originally proposed to be between 100 and 200 microns thick, but the stiffness requirements have pushed the needed thickness to the 300–800 micron range. A number of problems result from this thickening. First, pellicles thicker than approximately 200 microns induce spherical aberrations in the lithographic optics requiring correction, which adds complexity and/or costs. With these corrections built into the optics, a mask without the correct pellicle thickness, or with no pellicle, cannot be used in the same tool. This is disadvantageous because it is often desirable to use test masks without pellicles due to cost, turnaround time, and ease of handling. In addition, reticles used for tool calibration or setup, which do not have any requirements for low defect density, often are used with no pellicles. The requirement to use a pellicle in a tool which is pre-compensated for a particular pellicle thickness therefore increases the cost of test, calibration, setup, and experimental masks used with the tool. In addition, when spherical aberration corrections are added to the optics, the thick pellicle must also be built to tolerances similar to those of a lithographic lens element, since it must introduce the precise amount of spherical aberration that was corrected in the optics. This adds further cost. Consequently, thickening of the pellicle is not a sufficient remedy.
Another proposal is to use a removable pellicle, or no pellicle, and maintain control of particles to an extreme level in the tool and/or in-situ mask cleaning in the tool. Unfortunately, increased control also means increased costs, maintenance and time consumption. Accordingly, this proposal has not been widely embraced.
In view of the foregoing, there is a need a way to provide reduced pellicle distortion. It would be advantageous if the solution was applicable to both thin and thick pellicles.